i965/gs: Update defines related to GS surface organization.

i965/gs: Update defines related to GS surface organization.

Defines that previously referred to VS now refer to VEC4, since they
will be shared by the user-programmable vertex shader and geometry
shader stages.

Defines that previously referred to the Gen6 geometry shader stage
(which is only used for transform feedback) are now renamed to
explicitly refer to Gen6, to avoid confusion with the Gen7
user-programmable geometry shader stage.

Based on work by Eric Anholt <eric@anholt.net>.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>